It is now recognized that waterborne urethane/acrylic-or-vinyl emulsions can be prepared for use as coatings or adhesives with the advantage over earlier systems of being essentially free of volatile organic solvents. U.S. Pat. No. 4,644,030 issued to Loewrigkeit et al. teaches that these systems can be made by producing an isocyanate terminated polyurethane prepolymer in the presence of inert liquid polymerizable ethylenically unsaturated monomer, dispersing the prepolymer and monomer into water, chain extending the prepolymer, and subjecting the aqueous dispersion to vinyl addition polymerization conditions to polymerize the monomer. The use of the monomer as reactive diluent and solvent eliminates the need for an organic solvent medium and is claimed to result in intermixed or interpenetrated polymers. In the sequence followed in the process of this reference, the prepolymer polyurethane is dispersed into water and then chain extended. This sequence requires that the chain extender have hydrogens that are more reactive with isocyanate than does water. The chain extenders of choice for this purpose are typically and preferably organic polyamines. The result is a urethane polymer having both urethane and urea linkages.
U.S. Pat. No. 4,927,876 issued to Coogan et al. discloses a similar process and further discloses that the prepolymer polyurethane may be chain extended with water. As will be appreciated, when water is used as a chain extender, the result is a polymer having both urethane and urea linkages.
In general, urethane polymers containing no urea linkages will produce clearer films or coatings. It is assumed that this occurs because in some cases the presence of urea linkages could lead to crystallinity of the polymers, which translates to opacity in the resultant films or coatings. Therefore, there is a need for a method of preparing urethane/acrylic-or-vinyl emulsions in which the urethane polymer component contains no urea linkages.